Evolution of Structure and Chemistry of Bimetallic Nanoparticle Catalysts under Reaction Conditions

Abstract

Three series of bimetallic nanoparticle catalysts (Rh(x)Pd(1-x), Rh(x)Pt(1-x), and Pd(x)Pt(1-x), x = 0.2, 0.5, 0.8) were synthesized using one-step colloidal chemistry. X-ray photoelectron spectroscopy (XPS) depth profiles using different X-ray energies and scanning transmission electron microscopy showed that the as-synthesized Rh(x)Pd(1-x) and Pd(x)Pt(1-x) nanoparticles have a core-shell structure whereas the Rh(x)Pt(1-x) alloys are more homogeneous in structure. The evolution of their structures and chemistry under oxidizing and reducing conditions was studied with ambient-pressure XPS (AP-XPS) in the Torr pressure range. The Rh(x)Pd(1-x) and Rh(x)Pt(1-x) nanoparticles undergo reversible changes of surface composition and chemical state when the reactant gases change from oxidizing (NO or O(2) at 300 degrees C) to reducing (H(2) or CO at 300 degrees C) or catalytic (mixture of NO and CO at 300 degrees C). In contrast, no significant change in the distribution of the Pd and Pt atoms in the Pd(x)Pt(1-x) nanoparticles was observed. The difference in restructuring behavior under these reaction conditions in the three series of bimetallic nanoparticle catalysts is correlated with the surface free energy of the metals and the heat of formation of the metallic oxides. The observation of structural evolution of bimetallic nanoparticles under different reaction conditions suggests the importance of in situ studies of surface structures of nanoparticle catalysts.

Configurational Thermodynamics of Alloyed Nanoparticles with Adsorbates

“…8,9 We find that only core−shell NPs with compositions near 50% Rh have the core−shell reversal induced by O-coverage due to NP geometric size effect, agreeing with observation. 9 The simulated shell concentration of Rh in the small Pd 0.5 Rh 0.5 NP across the same effective O-coverage range also agrees with the experimental data on large NPs, reflecting the same origin from the relative strength of metal−O to metal−metal interactions.…”

“…8,9 We find that only core−shell NPs with compositions near 50% Rh have the core−shell reversal induced by O-coverage due to NP geometric size effect, agreeing with observation. 9 The simulated shell concentration of Rh in the small Pd 0.5 Rh 0.5 NP across the same effective O-coverage range also agrees with the experimental data on large NPs, reflecting the same origin from the relative strength of metal−O to metal−metal interactions. 8 Our study also reveals the detailed site occupations as functions of both the overall composition and O-coverage, providing design information for nanoalloy catalysts, especially the thermodynamically stable active sites under working conditions.…”

“…The improved catalytic properties have been attributed to many effects, such as new electronic states from alloying, Pd−Rh interfaces, and segregation behaviors induced by substrate and adsorbate. Among them, segregations induced by reaction condition were studied on thin films 27 and, more recently, on NPs to show a core−shell reversal 8,9 induced by adsorbates.…”

Exploring surface science and restructuring in reactive atmospheres of colloidally prepared bimetallic CuNi and CuCo nanoparticles on SiO2in situ using ambient pressure X-ray photoelectron spectroscopy

“…7 Lattice strain and relaxation effects were generally considered too small to be important. 6 In the present paper we focus on 10 exploring the changes on going from a bulk material to bimetallic metal nanoparticles, of which heterogeneous catalysts are normally comprised. This changes a number of important factors by essentially diminishing the importance of bulk energy terms relative to surface energy terms on the thermodynamic equilibrium structure of the particle.…”

Design of a new reactor-like high temperature near ambient pressure scanning tunneling microscope for catalysis studies

“…are very likely to restructure, in order to adapt their geometric and electronic structure to the surroundings. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] Such adaption induced from a gaseous or liquid environment could have profound effects on the functions of a catalyst. In many cases, the structure and composition of the catalyst surface in a reactive (or corrosive) environment in which it functions, differs from those in high vacuum.…”